US20070147542A1 - Method and apparatus for encoding a data stream for transmission across bonded channels - Google Patents
Method and apparatus for encoding a data stream for transmission across bonded channels Download PDFInfo
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- US20070147542A1 US20070147542A1 US11/316,129 US31612905A US2007147542A1 US 20070147542 A1 US20070147542 A1 US 20070147542A1 US 31612905 A US31612905 A US 31612905A US 2007147542 A1 US2007147542 A1 US 2007147542A1
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/32—Carrier systems characterised by combinations of two or more of the types covered by groups H04L27/02, H04L27/10, H04L27/18 or H04L27/26
- H04L27/34—Amplitude- and phase-modulated carrier systems, e.g. quadrature-amplitude modulated carrier systems
- H04L27/3405—Modifications of the signal space to increase the efficiency of transmission, e.g. reduction of the bit error rate, bandwidth, or average power
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
- H03M13/00—Coding, decoding or code conversion, for error detection or error correction; Coding theory basic assumptions; Coding bounds; Error probability evaluation methods; Channel models; Simulation or testing of codes
- H03M13/25—Error detection or forward error correction by signal space coding, i.e. adding redundancy in the signal constellation, e.g. Trellis Coded Modulation [TCM]
- H03M13/251—Error detection or forward error correction by signal space coding, i.e. adding redundancy in the signal constellation, e.g. Trellis Coded Modulation [TCM] with block coding
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L1/00—Arrangements for detecting or preventing errors in the information received
- H04L1/004—Arrangements for detecting or preventing errors in the information received by using forward error control
- H04L1/0041—Arrangements at the transmitter end
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L1/00—Arrangements for detecting or preventing errors in the information received
- H04L1/004—Arrangements for detecting or preventing errors in the information received by using forward error control
- H04L1/0045—Arrangements at the receiver end
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L1/00—Arrangements for detecting or preventing errors in the information received
- H04L1/004—Arrangements for detecting or preventing errors in the information received by using forward error control
- H04L1/0056—Systems characterized by the type of code used
- H04L1/0057—Block codes
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L1/00—Arrangements for detecting or preventing errors in the information received
- H04L1/004—Arrangements for detecting or preventing errors in the information received by using forward error control
- H04L1/0056—Systems characterized by the type of code used
- H04L1/0059—Convolutional codes
- H04L1/006—Trellis-coded modulation
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L1/00—Arrangements for detecting or preventing errors in the information received
- H04L1/004—Arrangements for detecting or preventing errors in the information received by using forward error control
- H04L1/0056—Systems characterized by the type of code used
- H04L1/0071—Use of interleaving
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
- H03M13/00—Coding, decoding or code conversion, for error detection or error correction; Coding theory basic assumptions; Coding bounds; Error probability evaluation methods; Channel models; Simulation or testing of codes
- H03M13/03—Error detection or forward error correction by redundancy in data representation, i.e. code words containing more digits than the source words
- H03M13/05—Error detection or forward error correction by redundancy in data representation, i.e. code words containing more digits than the source words using block codes, i.e. a predetermined number of check bits joined to a predetermined number of information bits
- H03M13/13—Linear codes
- H03M13/15—Cyclic codes, i.e. cyclic shifts of codewords produce other codewords, e.g. codes defined by a generator polynomial, Bose-Chaudhuri-Hocquenghem [BCH] codes
- H03M13/151—Cyclic codes, i.e. cyclic shifts of codewords produce other codewords, e.g. codes defined by a generator polynomial, Bose-Chaudhuri-Hocquenghem [BCH] codes using error location or error correction polynomials
- H03M13/1515—Reed-Solomon codes
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
- H03M13/00—Coding, decoding or code conversion, for error detection or error correction; Coding theory basic assumptions; Coding bounds; Error probability evaluation methods; Channel models; Simulation or testing of codes
- H03M13/27—Coding, decoding or code conversion, for error detection or error correction; Coding theory basic assumptions; Coding bounds; Error probability evaluation methods; Channel models; Simulation or testing of codes using interleaving techniques
- H03M13/2732—Convolutional interleaver; Interleavers using shift-registers or delay lines like, e.g. Ramsey type interleaver
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L1/00—Arrangements for detecting or preventing errors in the information received
- H04L2001/0092—Error control systems characterised by the topology of the transmission link
- H04L2001/0096—Channel splitting in point-to-point links
Definitions
- the present invention relates to content delivery systems and, more particularly, to a method and apparatus for encoding a data stream for transmission across bonded channels.
- Broadband content includes multiple types of communications and data, such as broadcast television channels, video-on-demand, streaming video, multimedia data, Internet access, packet telephony, etc.
- DSL digital subscriber line
- HFC hybrid fiber coaxial
- FEC forward error correction
- channel bonding To increase capacity, multiple downstream channels are being used to carry a single bitstream using a technique known as “channel bonding.”
- the input stream is multiplexed across multiple bonded channels at the transmitter and de-multiplexed at the receiver.
- Present channel bonding techniques used for downstream DOCSIS transmission multiplex data above the link layer by dividing a single protocol data unit (PDU) stream into a plurality of separate PDU streams.
- PDU protocol data unit
- Each of the separate PDU streams is then processed for FEC, modulated, and transmitted over a separate bonded channel. That is, the data on each of the bonded channels is separately FEC coded and QAM modulated, which exhibits some latency in the QAM forward channels.
- packets of the PDU stream are outer coded to produce a symbol stream.
- the symbol stream is formatted to produce trellis groups.
- the trellis groups are formed into blocks of M ⁇ N groups. For each of the blocks, M groups are processed using N trellis coders to produce N trellis coded outputs.
- Each of the N trellis encoder outputs is mapped onto a quadrature amplitude modulation (QAM) constellation to generate N QAM symbol streams.
- QAM quadrature amplitude modulation
- packets of the PDU stream are outer coded to produce a symbol stream.
- the symbol stream is formatted to produce trellis groups.
- the trellis groups are processed using a trellis coder to produce a trellis coded output.
- the trellis coded output is mapped onto a QAM constellation to generate a QAM symbol stream.
- the QAM symbol stream is formatted into N sets of QAM symbol groups to produce N QAM symbol streams.
- the N QAM symbol streams are modulated for transmission across N bonded channels.
- FIG. 1 is a block diagram depicting an exemplary embodiment of content encoding system in accordance with one or more aspects of the invention
- FIG. 2 is a flow diagram depicting an exemplary embodiment of a method of encoding a PDU stream in accordance with one or more aspects of the invention
- FIG. 3 is a block diagram depicting an exemplary embodiment of a decoder for decoding output of the encoding system of FIG. 1 ;
- FIG. 4 is a block diagram depicting another exemplary embodiment of a content encoding system in accordance with one or more aspects of the invention.
- FIG. 5 is a flow diagram depicting an exemplary embodiment of a method for encoding a PDU stream in accordance with one or more aspects of the invention.
- FIG. 6 is a block diagram depicting an exemplary embodiment of a decoder for decoding output of the encoding system of FIG. 4 .
- FIG. 1 is a block diagram depicting an exemplary embodiment of content encoding system 100 in accordance with one or more aspects of the invention.
- the encoding system 100 includes an outer code module 102 , a data formatter 104 , block forming logic 106 , trellis coders/QAM mappers 108 - 1 through 108 -N, and modulators 110 - 1 through 110 -N.
- the outer code module 102 includes a block coder 112 , an interleaver 114 , a sync preamble inserter 1 16 , and a randomizer 118 .
- the encoding system 100 is configured to process a stream of protocol data units (PDUs) to produce modulated data for downstream transmission over a plurality of bonded channels towards one or more decoders.
- PDUs protocol data units
- the encoding system 100 may be used to encode content for downstream transmission over bonded channels towards a modem in a data-over-cable service interface specification (DOCSIS) architecture.
- DOCSIS data-over-cable service interface specification
- the PDU stream may comprise transport stream packets, such as MPEG (moving picture experts group) transport stream packets. It is to be understood that the encoding system 100 may be configured to process other types of PDU streams known in the art.
- an input PDU stream is received by the block coder 112 .
- the block coder 112 applies a block code to the PDU packets in the input PDU stream.
- the outer code module 106 applies a Reed-Solomon (RS) block code.
- RS Reed-Solomon
- the block coded symbols produced by the block coder 112 are received by the interleaver 114 .
- the interleaver 114 comprises a, (I,J) convolutional interleaver.
- an (I,J) interleaver sequentially shifts the block coded symbols into a bank of I registers. Each successive register has J symbols more storage than the preceding register.
- the first interleaver path has zero delay, the second has J symbol period of delay, the third 2*J symbol periods of delay, and so on up to the 128 th path, which has 127*J symbol periods of delay.
- the interleaver 114 may be compliant with ITU J83.B.
- a single interleaving depth (128,1) may be used for 64-QAM.
- the interleaver 114 may employ variable interleaving (e.g., J may be an integer between 1 and 8).
- the interleaved symbols output by the interleaver 114 are received by the sync preamble inserter 116 .
- the sync preamble inserter 116 is configured to insert sync preambles for each FEC frame.
- Each sync preamble comprises a sequence of bits appended to the end of a group of blocks.
- the sync preamble combined with the group of blocks comprises an FEC frame.
- an FEC frame for 64-QAM includes a six symbol (42 bit) sync trailer appended to the end of 60 RS blocks, with each block including 128 symbols.
- An FEC frame for 256-QAM includes a 40-bit sync trailer appended to the end of 88 RS blocks. If the interleaver 114 employs variable interleaving, the type of interleaving is conveyed to the decoder using bits of the sync preamble.
- the symbol stream output by the sync preamble inserter 116 is received by the randomizer 118 .
- the randomizer 118 is initialized after each sync preamble and adds a pseudorandom noise (PN) sequence to the data symbols to generate a random sequence. Randomization is not applied to the sync preambles.
- the symbol stream output by the randomizer 118 is received by the data formatter 104 .
- the data formatter 104 processes the symbol stream to produce groups of bits referred to herein as “trellis groups”.
- Each trellis group includes un-coded bits and bits to be encoded. In one embodiment, each trellis group results in generation of five QAM symbols. The number of bits in each trellis group depends on the type of QAM used. In ITU J83.B, for 64-QAM, each trellis group includes 28 bits. Of the 28 bits, there are 20 un-coded bits and 8 bits to be encoded. Each 64-QAM symbol carries four randomized, interleaved, 7-bit RS code symbols.
- each of the first four QAM symbols in the group carries bits from each of the four 7-bit RS symbols, while the last QAM symbol carries bits from only two of the QAM symbols.
- an error to one 64-QAM symbol in the five-symbol trellis group at the demodulator can cause an error in all four RS symbols in the trellis group.
- two different types of trellis groups are defined: a non-sync group and a sync group.
- the non-sync group includes 38 data bits and the sync group includes 30 data bits and 8 sync bits. Of 38 bits in each trellis group, there are 30 un-coded bits and 8 bits to be encoded.
- bits are assigned sequentially to 256-QAM symbols in the trellis group from the serial randomized, interleaved RS codeword stream input.
- a given 256-QAM symbol carries bits spanning at most two 7-bit RS symbols.
- a channel error to one 256-QAM symbol may affect only two such RS symbols.
- the trellis codes are applied to the least-significant bits of the QAM I and Q portions of the symbol mapping; errors occurring in these bits of the QAM symbol can result in propagation of errors beyond a trellis group duration in bit times, due to the maximum likelihood trellis decoding (Viterbi algorithm) often employed in the receiver.
- the sequence of trellis groups produced by the data formatter 104 is received by the block forming logic 106 . Owing to the bit groupings within trellis groups described above for J83.B, it is advantageous to multiplex the randomizer output into trellis group blocks, as opposed to a finer multiplexing at the bit or QAM symbol level, for input to each of the N trellis encoder and QAM modulator blocks.
- the block forming logic 106 forms the trellis groups into blocks of M ⁇ N groups in preparation for subsequent trellis encoding and QAM mapping, where M is an integer and N corresponds the number N of trellis coders/QAM mappers 108 .
- the block forming logic 106 buffers the input trellis groups until N sets of M trellis groups are collected to form an M ⁇ N block of trellis groups.
- Set 1 includes the first (in time) M trellis groups
- set 2 includes the second (in time) M trellis groups
- set N includes the nth (in time) M trellis groups.
- the block forming logic 106 provides the N sets of M trellis groups to the N trellis coder/QAM mappers 108 - 1 through 108 -N, respectively. That is, each of the trellis coder/QAM mappers 108 receives a different set of M trellis groups. The process is repeated for each M ⁇ N block of trellis groups.
- the value M is chosen based on the expected duration of burst noise events on an individual QAM channel given the convolutional interleaver parameters and the known error correction capability of the outer RS code used in J.83B (for example).
- the (I,J) convolutional deinterleaver separates two consecutive input 7-bit RS symbols by IJ-1 RS symbol periods at the deinterleaver output.
- deinterleaver inputs that are separated in time by i symbols will be separated by i(IJ-1) symbols at the deinterleaver output.
- M can be chosen to be about equal the expected channel burst noise protection period covered by the choice of interleaver parameters, i.e., ⁇ 38M/7 ⁇ IJt.
- Each of the trellis coder/QAM mappers 108 is configured to perform trellis coding on each of the M trellis groups.
- each of the trellis coder/QAM mappers 108 encodes the bits to be encoded of each trellis group using, for example, a rate 4/5 convolutional coder on the least significant bits of the I and Q portions of the QAM symbol.
- the encoded bits are processed together with the un-coded bits to produce 5 QAM symbols. Since there are M trellis groups per M ⁇ N block, each of the trellis coder/QAM mappers 108 produces 5M QAM symbols.
- the trellis coding algorithm is well known in the art and is described in ITU J83.B.
- the trellis coder/QAM mappers 108 produce N QAM symbol streams.
- the QAM symbols produced by each of the trellis coder/QAM mappers 108 - 1 through 108 -N are received by the modulators 110 - 1 through 110 -N, respectively.
- the modulators 110 modulate the received QAM symbol streams on radio frequency (RF) carriers in a well known manner.
- the modulated RF carriers are configured for downstream transmission across N bonded channels.
- each of the outer code module 102 and the data formatter 104 is shared for each of the N bonded channels.
- the outer code module 102 operates at N times higher bit-rate than a similar outer code module would operate in a non-bonded encoding system.
- the interleaver/deinterleaver pair introduces a delay in the transmission system that is given by I(I ⁇ 1)J RS symbols in duration.
- FIG. 2 is a flow diagram depicting an exemplary embodiment of a method 200 of encoding a PDU stream in accordance with one or more aspects of the invention.
- the method 200 begins at step 202 , where sets of packets in the PDU stream are block coded to produce blocks of symbols.
- symbols are interleaved to produce interleaved symbols.
- sync preambles are inserted for each frame of interleaved symbols to generate a symbol stream.
- the symbol stream is randomized.
- the symbol stream is formatted to produce trellis groups.
- the trellis groups are formatted to produce N sets of M trellis groups.
- the N sets of M trellis groups are processed by N trellis coders to produce N trellis coded outputs.
- each of the N trellis coded outputs is mapped onto a QAM constellation to generate N QAM symbol streams.
- the N QAM symbol streams are modulated onto RF carriers and transmitted across N bonded channels. The method 200 is repeated for additional sets of packets in the PDU stream.
- FIG. 3 is a block diagram depicting an exemplary embodiment of a decoder 300 for decoding output of the encoding system 100 of FIG. 1 .
- the decoder 300 comprises demodulators 302 - 1 through 302 -N (collectively referred to as demodulators 302 ), trellis decoders 304 - 1 through 304 -N (collectively referred to as trellis decoders 304 ), a data re-combiner 306 , a de-randomizer 308 , a de-interleaver 310 , and a block decoder 312 .
- the demodulators 302 receive the modulated data from the encoding system 100 .
- Each of the demodulators 302 demodulates data from a respective one of the N bonded channels using well-known QAM demodulation techniques to produce in-phase and quadrature (I and Q) values. Each of the demodulators 302 produces one I, Q pair of values per symbol.
- the I, Q outputs of the demodulators 302 - 1 through 302 -N are respectively received by the trellis decoders 304 - 1 through 304 -N.
- Each of the trellis decoders 304 decodes the input I, Q pairs to produce a symbol stream.
- the N symbol streams produced by the trellis decoders 304 are received by the data re-combiner 306 .
- the data re-combiner 306 is configured to perform the inverse operation of the block forming logic 106 . That is, the data re-combiner 306 collects M symbol blocks for each of the N trellis decoders 304 and produces a single output symbol stream.
- the output symbol stream is received by the de-randomizer 308 .
- the de-randomizer 308 performs the inverse operation of the randomizer 118 .
- the output of the de-randomizer 308 is received by the de-interleaver 310 , which performs the inverse operation of the interleaver 114 .
- Output of the de-interleaver 310 is processed by the block decoder 312 , which performs the inverse operation of the block coder 112 .
- FIG. 4 is a block diagram depicting another exemplary embodiment of a content encoding system 400 in accordance with one or more aspects of the invention. Elements of FIG. 4 that are the same or similar to elements of FIG. 1 are designated with identical reference numerals and described above.
- an input stream is processed by the outer code module 102 and the data formatter 104 .
- the trellis coder/QAM mapper 402 trellis codes each trellis group produced by the data formatter.
- the trellis coder/QAM mapper 402 encodes the bits to be encoded of each trellis group and processes the encoded bits together with the un-coded bits to produce QAM symbols.
- the trellis coder/QAM mapper 402 employs a rate 4/5 convolutional encoder, and produces 5 QAM symbols per trellis group.
- the QAM symbols produce by the trellis coder/QAM mapper 402 are received by the block forming logic 404 .
- the block forming logic 404 collects N sets of QAM symbols and multiplexes the N sets among N outputs to produce N QAM symbol streams.
- the N QAM symbol streams are processed by the modulators 110 - 1 through 110 -N for modulation and transmission across N bonded channels.
- FIG. 6 is block diagram depicting an exemplary embodiment of a decoder 600 for decoding output of the encoding system 400 of FIG. 4 .
- the decoder 600 comprises demodulators 601 - 1 through 401 -N (collectively referred to as demodulators 601 ), a symbol re-combiner 602 , a trellis decoder 604 , a de-randomizer 606 , a de-interleaver 608 , and a block decoder 610 .
- the demodulators 601 receive the modulated data from the encoding system 400 .
- Each of the demodulators 602 demodulates data from a respective one of the N bonded channels using well-known QAM demodulation techniques to produce in-phase and quadrature (I and Q) values.
- Each of the demodulators 601 produces one I, Q pair of values per symbol.
- the I, Q outputs of the demodulators 601 - 1 through 601 -N are respectively received by the symbol re-combiner 602 .
- the symbol re-combiner 602 performs the inverse operation of the block forming logic 404 . That is, the symbol re-combiner 602 combines the I, Q pairs from each of the demodulators 601 to produce a single stream of I, Q pairs.
- the single stream of I, Q pairs is received by the trellis decoder 604 .
- the trellis decoder 604 decodes the input I, Q pairs to produce a symbol stream.
- the symbol stream produced by the trellis decoder 604 is received by the de-randomizer 606 .
- the de-randomizer 606 performs the inverse operation of the randomizer 118 .
- the output of the de-randomizer 606 is received by the de-interleaver 608 , which performs the inverse operation of the interleaver 114 .
- Output of the de-interleaver 608 is processed by the block decoder 610 , which performs the inverse operation of the block coder 112 .
- the encoding system 400 requires alignment of in-phase (I) and quadrature (Q) phase states (0, 90, 180, and 270 degree) in the N QAM demodulators 601 of the bonded channel decoder. If these phase states are not all the same, then the shared trellis decoder 604 would cause errors at channel bonding QAM symbol boundaries as the decoder input would experience bit inversions from QAM symbols demodulated in different phase states and would make errors until the inverted codeword bits were found and restored to the proper phase state. Phase misalignment may be detected by the shared trellis decoder 604 in low noise conditions (e.g., through decoder normalization rate).
- the encoding system 400 may include a QAM preamble inserter 406 .
- the QAM preamble inserter 406 is configured to insert known QAM symbol-based preambles simultaneously in each of the bonded QAM symbols streams.
- the demodulators 601 may use the QAM preambles to determine I, Q phase state.
- FIG. 5 is a flow diagram depicting an exemplary embodiment of a method 500 for encoding a PDU stream in accordance with one or more aspects of the invention. Steps of FIG. 5 that are the same or similar to steps in FIG. 2 are designated with identical reference numerals and described above.
- the method 500 proceeds to step 502 .
- the trellis groups are processed by a trellis coder to produce a trellis coded output.
- the trellis coded output is mapped onto a QAM constellation to produce a QAM symbol stream.
- the QAM symbol stream is formatted to produce N QAM symbol streams.
- step 507 QAM preambles are inserted into each of the N QAM symbol streams.
- step 508 the N QAM symbol streams are modulated for transmission over N bonded channels. The method 500 is repeated for additional sets of packets in the PDU stream.
Abstract
Description
- 1. Field of the Invention
- The present invention relates to content delivery systems and, more particularly, to a method and apparatus for encoding a data stream for transmission across bonded channels.
- 2. Description of the Background Art
- The demand for broadband content by business and residential subscribers is continually increasing. Broadband content includes multiple types of communications and data, such as broadcast television channels, video-on-demand, streaming video, multimedia data, Internet access, packet telephony, etc. To meet the increasing demand, it is typically necessary to increase throughput to each subscriber and improve overall quality of service. Current delivery technologies include several variations of digital subscriber line (DSL) technology, which uses telephony facilities, and cable modem systems using cable television facilities and hybrid fiber coaxial (HFC) distribution networks.
- Delivery of data services over cable television systems is typically compliant with the Data-over-cable-service-interface-specifications (DOCSIS) standard. The content is typically modulated using quadrature amplitude modulation (QAM). Current cable QAM standards use conventional forward error correction (FEC) techniques to transmit the data downstream. FEC is a system of error control for data transmission where the receiving device has the capability to detect and correct fewer than a predetermined number or fraction of bits or symbols corrupted by transmission errors. FEC is accomplished by adding redundancy to the transmitted information using a predetermined algorithm. The original information may or may not appear in the encoded output; codes that include the un-modified input in the output are systematic, while those that do not are nonsystematic.
- To increase capacity, multiple downstream channels are being used to carry a single bitstream using a technique known as “channel bonding.” The input stream is multiplexed across multiple bonded channels at the transmitter and de-multiplexed at the receiver. Present channel bonding techniques used for downstream DOCSIS transmission multiplex data above the link layer by dividing a single protocol data unit (PDU) stream into a plurality of separate PDU streams. Each of the separate PDU streams is then processed for FEC, modulated, and transmitted over a separate bonded channel. That is, the data on each of the bonded channels is separately FEC coded and QAM modulated, which exhibits some latency in the QAM forward channels.
- Accordingly, there exists a need in the art for a method and apparatus that multiplexing forward channels across bonded channels with reduced latency.
- Method and apparatus for encoding a protocol data unit (PDU) stream is described. In one embodiment, packets of the PDU stream are outer coded to produce a symbol stream. The symbol stream is formatted to produce trellis groups. The trellis groups are formed into blocks of M×N groups. For each of the blocks, M groups are processed using N trellis coders to produce N trellis coded outputs. Each of the N trellis encoder outputs is mapped onto a quadrature amplitude modulation (QAM) constellation to generate N QAM symbol streams.
- In another embodiment, packets of the PDU stream are outer coded to produce a symbol stream. The symbol stream is formatted to produce trellis groups. The trellis groups are processed using a trellis coder to produce a trellis coded output. The trellis coded output is mapped onto a QAM constellation to generate a QAM symbol stream. the QAM symbol stream is formatted into N sets of QAM symbol groups to produce N QAM symbol streams. The N QAM symbol streams are modulated for transmission across N bonded channels.
- So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
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FIG. 1 is a block diagram depicting an exemplary embodiment of content encoding system in accordance with one or more aspects of the invention; -
FIG. 2 is a flow diagram depicting an exemplary embodiment of a method of encoding a PDU stream in accordance with one or more aspects of the invention; -
FIG. 3 is a block diagram depicting an exemplary embodiment of a decoder for decoding output of the encoding system ofFIG. 1 ; -
FIG. 4 is a block diagram depicting another exemplary embodiment of a content encoding system in accordance with one or more aspects of the invention; -
FIG. 5 is a flow diagram depicting an exemplary embodiment of a method for encoding a PDU stream in accordance with one or more aspects of the invention; and -
FIG. 6 is a block diagram depicting an exemplary embodiment of a decoder for decoding output of the encoding system ofFIG. 4 . - To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures.
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FIG. 1 is a block diagram depicting an exemplary embodiment ofcontent encoding system 100 in accordance with one or more aspects of the invention. Theencoding system 100 includes anouter code module 102, adata formatter 104,block forming logic 106, trellis coders/QAM mappers 108-1 through 108-N, and modulators 110-1 through 110-N. Theouter code module 102 includes ablock coder 112, aninterleaver 114, async preamble inserter 1 16, and arandomizer 118. Theencoding system 100 is configured to process a stream of protocol data units (PDUs) to produce modulated data for downstream transmission over a plurality of bonded channels towards one or more decoders. For example, theencoding system 100 may be used to encode content for downstream transmission over bonded channels towards a modem in a data-over-cable service interface specification (DOCSIS) architecture. The PDU stream may comprise transport stream packets, such as MPEG (moving picture experts group) transport stream packets. It is to be understood that theencoding system 100 may be configured to process other types of PDU streams known in the art. - In particular, an input PDU stream is received by the
block coder 112. In one embodiment, theblock coder 112 applies a block code to the PDU packets in the input PDU stream. In one embodiment, theouter code module 106 applies a Reed-Solomon (RS) block code. For example, theblock coder 112 may apply an (n,k)=(128,122), t=3 symbol-error correcting RS code over Galois Field GF(128) to each PDU packet. That is, theblock coder 112 produces 128-symbol blocks having six parity symbols, where each of the symbols is seven bits. Bit errors occurring within three or less symbols of a block at the decoder can be corrected. Details of a (128,122), t=3 RS code are described in the specification of the International Telecommunications Union (ITU) Recommendation J.83 Annex B (referred to as ITU J83.B). - The block coded symbols produced by the
block coder 112 are received by theinterleaver 114. In one embodiment, theinterleaver 114 comprises a, (I,J) convolutional interleaver. As is well known in the art, an (I,J) interleaver sequentially shifts the block coded symbols into a bank of I registers. Each successive register has J symbols more storage than the preceding register. For example, theinterleaver 114 may have a depth of I=128. The first interleaver path has zero delay, the second has J symbol period of delay, the third 2*J symbol periods of delay, and so on up to the 128th path, which has 127*J symbol periods of delay. If burst noise in the channel causes a series of bad symbols, they are spread over many symbol blocks (e.g., RS blocks) by a deinterleaver in the decoder such that the resultant symbol errors per block are within the range of the block decoder correction capability. For example, theinterleaver 114 may be compliant with ITU J83.B. In particular, a single interleaving depth (128,1) may be used for 64-QAM. For 64-QAM or 256-QAM, theinterleaver 114 may employ variable interleaving (e.g., J may be an integer between 1 and 8). - The interleaved symbols output by the
interleaver 114 are received by thesync preamble inserter 116. Thesync preamble inserter 116 is configured to insert sync preambles for each FEC frame. Each sync preamble comprises a sequence of bits appended to the end of a group of blocks. The sync preamble combined with the group of blocks comprises an FEC frame. For example, in ITU J83.B, an FEC frame for 64-QAM includes a six symbol (42 bit) sync trailer appended to the end of 60 RS blocks, with each block including 128 symbols. An FEC frame for 256-QAM includes a 40-bit sync trailer appended to the end of 88 RS blocks. If theinterleaver 114 employs variable interleaving, the type of interleaving is conveyed to the decoder using bits of the sync preamble. - The symbol stream output by the
sync preamble inserter 116 is received by therandomizer 118. Therandomizer 118 is initialized after each sync preamble and adds a pseudorandom noise (PN) sequence to the data symbols to generate a random sequence. Randomization is not applied to the sync preambles. Therandomizer 118 may use a linear feedback shift register specified by a polynomial over GF(128) defined as follows:
f(x)=x 3 +x+α
Where α is a primitive element of GF(128) such that α7+α3+1=0. - The symbol stream output by the
randomizer 118 is received by thedata formatter 104. The data formatter 104 processes the symbol stream to produce groups of bits referred to herein as “trellis groups”. Each trellis group includes un-coded bits and bits to be encoded. In one embodiment, each trellis group results in generation of five QAM symbols. The number of bits in each trellis group depends on the type of QAM used. In ITU J83.B, for 64-QAM, each trellis group includes 28 bits. Of the 28 bits, there are 20 un-coded bits and 8 bits to be encoded. Each 64-QAM symbol carries four randomized, interleaved, 7-bit RS code symbols. However, the grouping is such that each of the first four QAM symbols in the group carries bits from each of the four 7-bit RS symbols, while the last QAM symbol carries bits from only two of the QAM symbols. Hence, an error to one 64-QAM symbol in the five-symbol trellis group at the demodulator can cause an error in all four RS symbols in the trellis group. For 256-QAM, two different types of trellis groups are defined: a non-sync group and a sync group. The non-sync group includes 38 data bits and the sync group includes 30 data bits and 8 sync bits. Of 38 bits in each trellis group, there are 30 un-coded bits and 8 bits to be encoded. These bits are assigned sequentially to 256-QAM symbols in the trellis group from the serial randomized, interleaved RS codeword stream input. Thus, a given 256-QAM symbol carries bits spanning at most two 7-bit RS symbols. A channel error to one 256-QAM symbol may affect only two such RS symbols. However, note that in both 64-QAM and 256-QAM, the trellis codes are applied to the least-significant bits of the QAM I and Q portions of the symbol mapping; errors occurring in these bits of the QAM symbol can result in propagation of errors beyond a trellis group duration in bit times, due to the maximum likelihood trellis decoding (Viterbi algorithm) often employed in the receiver. - The sequence of trellis groups produced by the data formatter 104 is received by the
block forming logic 106. Owing to the bit groupings within trellis groups described above for J83.B, it is advantageous to multiplex the randomizer output into trellis group blocks, as opposed to a finer multiplexing at the bit or QAM symbol level, for input to each of the N trellis encoder and QAM modulator blocks. Theblock forming logic 106 forms the trellis groups into blocks of M×N groups in preparation for subsequent trellis encoding and QAM mapping, where M is an integer and N corresponds the number N of trellis coders/QAM mappers 108. That is, theblock forming logic 106 buffers the input trellis groups until N sets of M trellis groups are collected to form an M×N block of trellis groups.Set 1 includes the first (in time) M trellis groups, set 2 includes the second (in time) M trellis groups, and so on until set N includes the nth (in time) M trellis groups. After an M×N block of trellis groups is collected, theblock forming logic 106 provides the N sets of M trellis groups to the N trellis coder/QAM mappers 108-1 through 108-N, respectively. That is, each of the trellis coder/QAM mappers 108 receives a different set of M trellis groups. The process is repeated for each M×N block of trellis groups. - The value M is chosen based on the expected duration of burst noise events on an individual QAM channel given the convolutional interleaver parameters and the known error correction capability of the outer RS code used in J.83B (for example). As is well known in the art, the (I,J) convolutional deinterleaver separates two consecutive input 7-bit RS symbols by IJ-1 RS symbol periods at the deinterleaver output. Similarly, deinterleaver inputs that are separated in time by i symbols will be separated by i(IJ-1) symbols at the deinterleaver output. For a t=3 symbol-error-correcting, length n=128 symbol outer RS code such as in J83.B, three symbol errors occurring in any length n input can be corrected by the RS decoder. By choosing I=n=128, a burst of errored symbols at the deinterleaver input that is shorter than IJt symbols will have less than t=3 errors occurring in any n=128 symbol deinterleaver output word and, hence, will be corrected by the RS decoder (given proper deinterleaver synchronization and no other errors at the interleaver input than those caused by the burst). Other regular symbol error patterns at the deinterleaver input will also result in correctable output codewords, e.g., if every fourth RS symbol was in error at this deinterleaver input over a span of 4IJt symbols. In one embodiment, each trellis encoder/QAM mapper is encoding a sequential block of 28M bits carrying 28M/7=4M RS symbols or 38M bits carrying 38M/7 RS symbols for J83.B 64-QAM or 256-QAM, respectively. Hence, M can be chosen to be about equal the expected channel burst noise protection period covered by the choice of interleaver parameters, i.e., ┌38M/7┐≈IJt.
- Each of the trellis coder/
QAM mappers 108 is configured to perform trellis coding on each of the M trellis groups. In particular, each of the trellis coder/QAM mappers 108 encodes the bits to be encoded of each trellis group using, for example, a rate 4/5 convolutional coder on the least significant bits of the I and Q portions of the QAM symbol. As such, for each trellis group, the encoded bits are processed together with the un-coded bits to produce 5 QAM symbols. Since there are M trellis groups per M×N block, each of the trellis coder/QAM mappers 108 produces 5M QAM symbols. The trellis coding algorithm is well known in the art and is described in ITU J83.B. In this manner, the trellis coder/QAM mappers 108 produce N QAM symbol streams. The QAM symbols produced by each of the trellis coder/QAM mappers 108-1 through 108-N are received by the modulators 110-1 through 110-N, respectively. Themodulators 110 modulate the received QAM symbol streams on radio frequency (RF) carriers in a well known manner. The modulated RF carriers are configured for downstream transmission across N bonded channels. - In this manner, each of the
outer code module 102 and the data formatter 104 is shared for each of the N bonded channels. For N bonded channels, theouter code module 102 operates at N times higher bit-rate than a similar outer code module would operate in a non-bonded encoding system. Consider a four channel bonding group (N=4) and a desired burst protection level of 66 microseconds for a J83.B 256-QAM system. For the 256-QAM symbol rate of 5.36 Msps, a burst length of 66 μsec affects (66×10−6)(5.36×106)≈354 QAM symbols which represents 354/5=70.8 trellis groups each containing 38/7 RS code symbols, i.e., (70.8)(38/7) 384 RS symbols. Assuming only one of the four QAM channels experienced burst noise in any reasonable burst repetition period (e.g., tens of milliseconds), then the interleaver 114 (which is shared among the N bonded channels) need only protect up to 384 RS symbols, i.e., I=128, J=1 interleaving. This would imply a blocking of M˜┌(384×7)/38┐=71 38-bit groups per trellis coder/QAM mapper input. The interleaver/deinterleaver pair would be operating at N=4 times higher rate than non-bonded operation, resulting in a latency of (2.8 milliseconds)/4=0.7 milliseconds. The blocking and buffering of the M=71 38-bit trellis groups would impose an additional delay of 66 microseconds at the transmitter, so the overall delay is 0.766 milliseconds. - In a non-bonded encoder, the interleaver/deinterleaver pair introduces a delay in the transmission system that is given by I(I−1)J RS symbols in duration. For I=128, J=1, this corresponds to 2.8 milliseconds. For I=128, J=8, this corresponds to 22.3 milliseconds. Thus, the
encoding system 100 of the invention reduces latency from 2.8 milliseconds for a non-bonded encoder to 0.77 milliseconds for I=128, J=1 interleaving. For I=128, J=8 interleaving, theencoding system 100 reduces latency from 22 milliseconds for a non-bonded encoder to (22 milliseconds)/8+0.5 milliseconds=3.25 milliseconds. -
FIG. 2 is a flow diagram depicting an exemplary embodiment of amethod 200 of encoding a PDU stream in accordance with one or more aspects of the invention. Themethod 200 begins atstep 202, where sets of packets in the PDU stream are block coded to produce blocks of symbols. Atstep 204, symbols are interleaved to produce interleaved symbols. Atstep 206, sync preambles are inserted for each frame of interleaved symbols to generate a symbol stream. Atstep 208, the symbol stream is randomized. Atstep 210, the symbol stream is formatted to produce trellis groups. Atstep 212, the trellis groups are formatted to produce N sets of M trellis groups. Atstep 214, the N sets of M trellis groups are processed by N trellis coders to produce N trellis coded outputs. Atstep 216, each of the N trellis coded outputs is mapped onto a QAM constellation to generate N QAM symbol streams. Atstep 218, the N QAM symbol streams are modulated onto RF carriers and transmitted across N bonded channels. Themethod 200 is repeated for additional sets of packets in the PDU stream. -
FIG. 3 is a block diagram depicting an exemplary embodiment of adecoder 300 for decoding output of theencoding system 100 ofFIG. 1 . Thedecoder 300 comprises demodulators 302-1 through 302-N (collectively referred to as demodulators 302), trellis decoders 304-1 through 304-N (collectively referred to as trellis decoders 304), adata re-combiner 306, a de-randomizer 308, a de-interleaver 310, and ablock decoder 312. Thedemodulators 302 receive the modulated data from theencoding system 100. Each of thedemodulators 302 demodulates data from a respective one of the N bonded channels using well-known QAM demodulation techniques to produce in-phase and quadrature (I and Q) values. Each of thedemodulators 302 produces one I, Q pair of values per symbol. - The I, Q outputs of the demodulators 302-1 through 302-N are respectively received by the trellis decoders 304-1 through 304-N. Each of the
trellis decoders 304 decodes the input I, Q pairs to produce a symbol stream. The N symbol streams produced by thetrellis decoders 304 are received by thedata re-combiner 306. The data re-combiner 306 is configured to perform the inverse operation of theblock forming logic 106. That is, the data re-combiner 306 collects M symbol blocks for each of theN trellis decoders 304 and produces a single output symbol stream. The output symbol stream is received by the de-randomizer 308. The de-randomizer 308 performs the inverse operation of therandomizer 118. The output of the de-randomizer 308 is received by the de-interleaver 310, which performs the inverse operation of theinterleaver 114. Output of the de-interleaver 310 is processed by theblock decoder 312, which performs the inverse operation of theblock coder 112. -
FIG. 4 is a block diagram depicting another exemplary embodiment of acontent encoding system 400 in accordance with one or more aspects of the invention. Elements ofFIG. 4 that are the same or similar to elements ofFIG. 1 are designated with identical reference numerals and described above. In the present embodiment, an input stream is processed by theouter code module 102 and thedata formatter 104. The trellis coder/QAM mapper 402 trellis codes each trellis group produced by the data formatter. The trellis coder/QAM mapper 402 encodes the bits to be encoded of each trellis group and processes the encoded bits together with the un-coded bits to produce QAM symbols. In one embodiment, the trellis coder/QAM mapper 402 employs a rate 4/5 convolutional encoder, and produces 5 QAM symbols per trellis group. The QAM symbols produce by the trellis coder/QAM mapper 402 are received by theblock forming logic 404. Theblock forming logic 404 collects N sets of QAM symbols and multiplexes the N sets among N outputs to produce N QAM symbol streams. The N QAM symbol streams are processed by the modulators 110-1 through 110-N for modulation and transmission across N bonded channels. -
FIG. 6 is block diagram depicting an exemplary embodiment of adecoder 600 for decoding output of theencoding system 400 ofFIG. 4 . Thedecoder 600 comprises demodulators 601-1 through 401-N (collectively referred to as demodulators 601), asymbol re-combiner 602, atrellis decoder 604, a de-randomizer 606, a de-interleaver 608, and ablock decoder 610. Thedemodulators 601 receive the modulated data from theencoding system 400. Each of thedemodulators 602 demodulates data from a respective one of the N bonded channels using well-known QAM demodulation techniques to produce in-phase and quadrature (I and Q) values. Each of thedemodulators 601 produces one I, Q pair of values per symbol. - The I, Q outputs of the demodulators 601-1 through 601-N are respectively received by the
symbol re-combiner 602. Thesymbol re-combiner 602 performs the inverse operation of theblock forming logic 404. That is, thesymbol re-combiner 602 combines the I, Q pairs from each of thedemodulators 601 to produce a single stream of I, Q pairs. The single stream of I, Q pairs is received by thetrellis decoder 604. Thetrellis decoder 604 decodes the input I, Q pairs to produce a symbol stream. The symbol stream produced by thetrellis decoder 604 is received by the de-randomizer 606. The de-randomizer 606 performs the inverse operation of therandomizer 118. The output of the de-randomizer 606 is received by the de-interleaver 608, which performs the inverse operation of theinterleaver 114. Output of the de-interleaver 608 is processed by theblock decoder 610, which performs the inverse operation of theblock coder 112. - The
encoding system 400 requires alignment of in-phase (I) and quadrature (Q) phase states (0, 90, 180, and 270 degree) in theN QAM demodulators 601 of the bonded channel decoder. If these phase states are not all the same, then the sharedtrellis decoder 604 would cause errors at channel bonding QAM symbol boundaries as the decoder input would experience bit inversions from QAM symbols demodulated in different phase states and would make errors until the inverted codeword bits were found and restored to the proper phase state. Phase misalignment may be detected by the sharedtrellis decoder 604 in low noise conditions (e.g., through decoder normalization rate). Alternatively, theencoding system 400 may include aQAM preamble inserter 406. TheQAM preamble inserter 406 is configured to insert known QAM symbol-based preambles simultaneously in each of the bonded QAM symbols streams. Thedemodulators 601 may use the QAM preambles to determine I, Q phase state. -
FIG. 5 is a flow diagram depicting an exemplary embodiment of amethod 500 for encoding a PDU stream in accordance with one or more aspects of the invention. Steps ofFIG. 5 that are the same or similar to steps inFIG. 2 are designated with identical reference numerals and described above. Afterstep 210, themethod 500 proceeds to step 502. At step 502, the trellis groups are processed by a trellis coder to produce a trellis coded output. Atstep 504, the trellis coded output is mapped onto a QAM constellation to produce a QAM symbol stream. Atstep 506, the QAM symbol stream is formatted to produce N QAM symbol streams. Atoptional step 507, QAM preambles are inserted into each of the N QAM symbol streams. Atstep 508, the N QAM symbol streams are modulated for transmission over N bonded channels. Themethod 500 is repeated for additional sets of packets in the PDU stream. - While the foregoing is directed to illustrative embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.
Claims (20)
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US11/316,129 US20070147542A1 (en) | 2005-12-22 | 2005-12-22 | Method and apparatus for encoding a data stream for transmission across bonded channels |
CA002571749A CA2571749A1 (en) | 2005-12-22 | 2006-12-19 | Method and apparatus for encoding a data stream for transmission across bonded channels |
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